1. Field of the Invention
The present invention relates to a ceramic package for accomodating semiconductor dies, and more particularly to such a ceramic package which may be heat-sealed with a ceramic cover by glass of relatively low melting point. The present invention can be advantageously applied to produce a highly reliable package having excellent characteristics, including high thermal conductivity, high density mounting, hermetic sealing, and so on.
2. Description of the Prior Art
A ceramic package may be made by forming conductive patterns of tungsten paste or the like on alumina layers, then laminating and baking the latter. Thereafter, outerleads may be brazed and surface treatment using a plating process may be carried out in order to give the required qualities of die and wire-bonding, hermetic sealing, soldering, and so on. A heretofore known surface finishing process for making a ceramic package includes, first, nickel plating on the metalized layers for improving brazing quality of the outerleads, second, electrolytic nickel plating of 2.0 .mu.m to 3.0 .mu.m after the brazing, the gold plating about 2.0 .mu.m thick on the nickel. The compositions of metal to be plated and thicknesses thereof may be determined according to the heating process during assembly of the semiconductor packages. Consequently, the packages are subjected to a heating processes, such as die-bonding at 450.degree. C. for five minutes; wire-bonding of 300.degree. C. to 350.degree. C. for five minutes; hermetic heat-sealing with metal lids at 350.degree. C. for five minutes (where using Au/Sn eutectic alloy), or hermetic heat-sealing with ceramic covers at 480.degree. C. for 12 minutes (where using glass of low melting point). In addition, in the case of highly integrated memories, after being assembled, semiconductor elements are subjected to an aging process of 150.degree. C. for 24 hours. Therefore, it is required that the packages be sufficiently heatproof to preserve various functional qualities during the assembly processes.
In a conventional ceramic package plated with nickel 3.0 .mu.m thick and gold 2.0 .mu.m thick, heat tests of 450.degree. C. for five minutes, sometimes show the fact that nickel diffuses onto the gold surface and forms an oxide. One of the reasons is that the metalized surface of the underlayer of nickel is so rough. Therefore, expensive gold must sometimes be plated relatively thickly.
However, it is nowadays required that the thickness of the gold plated layer be thin and that gold be plated only on selective areas, i.e., bonding areas, such as the die and wire-bonding areas, preserve the functional qualities of the conventional art, in view of reducing cost. In the case of plating nickel and annealing at 750.degree. C., ten minutes is generally necessary before gold is plated, otherwise there is a possibility that the dies will peel off. If the nickel cannot be annealed, the plated gold is poor in appearance and the grains thereof are so small, the underlayer of nickel is easily heat-diffused. Therefore, nickel and gold plated packages do not have completely sufficient thermal qualities with respect to the heating conditions during assembly. As a result the qualities depending on heat resistability, such as qualities of die-bonding, wire-bonding and soldering, are unstable. Particularly, a selective gold plated package wherein no gold is plated on the outerlead portions is heated 450.degree. C. to 480.degree. C. when a die is bonded and when a cover is heat-sealed by glass having a low melting point. On the nickel surfaces of the outerlead portions, obstinate oxide films will be formed, which cannot easily be removed by flux which may be used during a subsequent soldering process. Therefore, functionally superior packages cannot be obtained through a soldering process unless a special acid treatment is performed, because the characteristic of solder wetting is poor due to the oxide film.